In the case of electrostatic discharges, ESD, high voltages occur for example between terminals of an electrical circuit. This can lead to high currents through the circuit. In the case of integrated circuits, in particular, it is possible for the circuit to be destroyed thereby.
In order to protect a circuit against electrostatic discharges, it is possible to provide protection circuits which, in the case of an electrostatic discharge, that is to say when a high voltage occurs, can divert a current and thus protect the electrical circuit against being destroyed.
FIG. 11 shows an exemplary embodiment of a conventional circuit for protection against electrostatic discharges for positive signal voltages. A PNP bipolar transistor T1 is connected as a diverting element between terminals K1, K2. In addition, the terminals K1, K2 are coupled via a series circuit comprising a resistor R1 and a Zener diode D1. The connecting node between the components in the series circuit is connected to the control terminal or base terminal B of the transistor T1.
Particularly in the case of circuits in the semiconductor area which operate with relatively high signal voltages or operating voltages, PNP transistors are often used as diverting elements in protection circuits since, in contrast to NPN transistors, they do not have a voltage snapback. A diverting element having voltage snapback goes into a low-impedance state after triggering and causes the voltage present across the diverting element to fall. This is shown by way of example in a voltage-current diagram of a diverting element in FIG. 12. In the diagram, a current I through a diverting element is represented as a function of a voltage V across the diverting element. Proceeding from a low voltage, the diverting element attains a conducting state in the event of an increase in the voltage to or above a breakdown voltage VBP, whereby a current starts to flow. At the same time, however, the voltage across the diverting element firstly falls to a holding voltage VH. Current and voltage can rise further from this point on.
In applications with lower operating voltages, for example an operating voltage VNV, the holding voltage VH lies above the operating voltage VNV of the circuit arrangement and does not lead to a rise in the current through the diverting element. At higher operating voltages, the holding voltage VH may lie below a different operating voltage VHV, for example. In the event of triggering of a diverting element having a voltage snapback, the supply voltage VHV may be present across the low-impedance diverting element and bring about a high current flow that may lead as far as the destruction of the diverting element.
If, in FIG. 11, the voltage between emitter E and collector C of the transistor T1 exceeds the Zener voltage of the Zener diode D1, a current can flow from the base of the transistor T1, whereby the transistor T1 is turned on and starts to conduct. However, a Zener diode generally supplies relatively little triggering current for turning on the transistor. The Zener diode D1 should therefore be given sufficiently large dimensions in order to supply a necessary triggering current for a reliable turn-on of the transistor T1.
FIG. 13 shows a further exemplary embodiment of a conventional circuit for protection against electrostatic discharges for positive and negative signal voltages. The PNP transistor T1 is connected between the terminals K1, K2. The base terminal B of the transistor T1 is not connected. In order that a circuit arrangement of this type can be used for positive and negative signal voltages, it is expedient that the base B of the transistor T1 can float with regard to its potential, that is to say is not connected to a fixed potential. This is intended to have the effect that an internal diode of the bipolar transistor T1 is forward-biased neither in the positive direction nor in the negative direction.
In normal operation, that is to say where no electrostatic discharge occurs, the transistor T1 should be switched off, that is to say non-conducting. In the case of an electrostatic discharge, the voltage between emitter E and collector C of the transistors T1 exceeds a normal operating voltage of the transistor, whereby it becomes conducting. By way of example, in the case of a positive pulse of an electrostatic discharge between emitter E and collector C, the PN junction between base B and collector C can break down and thus turn on the transistor T1. As an alternative, by way of example, in the case of a negative pulse of an electrostatic discharge, the PN junction between base B and emitter E can break down, whereby the transistor T1 is turned on. However, the base current generated by the pulse of the electrostatic discharge in the transistor T1 is usually small, such that the effectiveness of an arrangement of this type is generally inadequate.
Consequently, the arrangements described cannot ensure that a diverting element is reliably turned on. Accordingly, a current caused by an overvoltage on account of an electrostatic discharge cannot reliably be diverted from a circuit to be protected.